The present invention relates to a facsimile transceiver, printer or similar image forming apparatus.
An image forming apparatus of the type using an electrophotographic copying or image recording process, for example, electrostatically forms a latent image on a photoconductive element or similar image carrier and then develops it by a developing device to produce a toner image. It has been customary with this type of apparatus to fix the toner image directly on the image carrier, or to transfer the toner image from the image carrier to a recording medium, e.g., a paper sheet and then fix it on the paper sheet, or to transfer the toner image from the image carrier to a transfer belt or similar intermediate transfer body and then from the intermediate transfer body to a recording medium to fix it on the recording medium. Conventional developing devices for the above application include one which uses a two-component type developer, i.e., a mixture of toner and carrier and forms a bias electric field for development having an alternating electric field in a developing region where a developer carrier thereof faces the image carrier. In an image forming apparatus with this kind of developing device, the frequency of the alternating electric field may be changed in matching relation to the kind of a document, as disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 116365/1982. Alternatively, the frequency of the alternating electric field may be changed in association with the spatial frequency of a document, as proposed in Japanese Patent Laid-Open Publication No. 42070/1983. Further, the bias electric field may be controlled on the basis of the ambient conditions, as taught in Japanese Patent Laid-Open Publication No. 186369/1990.
However, by repetitively forming an image while observing the changes in image quality due to aging, we found that the image forming apparatus having a developing device of the type using a two-component type developer and developing a latent image by forming a bias electric field having an alternating electric field in a developing region has the following problems left unsolved. The degree of edge emphasis, e.g., the occurrence that the image density is higher at the edges than at the central portion is aggravated due to aging, resulting in poor image quality. It also occurs that the image density is lower at the edges than at the central portion and, in addition, the reproduced image has an undulatory contour, preventing the image from having sharpness. Either of the edge emphasis and the fall of sharpness is apt to occur, depending on the kind of carrier used.
Edge emphasis rarely occurs when a latent image is developed by a two-component type developer including a conductive carrier, while it is apt to occur when use is made of a two-component type developer including an insulative carrier having a higher resistance than the conductive carrier, as well known in the art. A conductive carrier, for example, renders the developing characteristic flat relative to the frequency of a latent image without accentuating particular frequencies. By contrast, an insulative carrier accentuates particular frequencies, i.e., edges. It is also known that the resistance of the carrier increases when the toner firmly adheres to the surface of the carrier due to aging, i.e., the carrier is spent. We, therefore, assumed that the edge emphasis due to aging is aggravated by the increase in the resistance of the carrier due to aging and conducted a series of experiments for determining the relation between the resistance of the carrier and the degree of edge emphasis by using carriers each having a particular resistance. The experiments showed that the degree of emphasis increases with the increase in the resistance of the carrier. On the other hand, it has been reported that films coating the insulative carrier are shaved off due to aging to lower the resistance of the carrier. This led us to a conclusion that the fall of sharpness due to aging results from the decrease in the resistance of the carrier due to aging.
By extended experiments and studies, we found that the edge emphasis and the fall of sharpness have a correlation with the peak-to-peak voltage and frequency of an AC component included in a voltage which is applied to a developing sleeve or similar counter electrode member, and that the peak-to-peak voltage and frequency each has an adequate range for producing an attractive image free from edge emphasis and the fall of sharpness. We further found that such adequate ranges depend on the resistance of the carrier. In addition, we found that the above-stated fall of sharpness occurs when the peak-to-peak voltage and the frequency are higher than the respective adequate ranges.
From the above, it follows that the peak-to-peak voltage and the frequency for minimizing edge emphasis and the fall of sharpness are brought out of their adequate ranges due to the change in the resistance of the carrier due to aging, sequentially lowering the image quality as a result of edge emphasis and the fall of sharpness.